In order to create a historical replay league you need to begin by downloading a community created historical database. Check the mods forum for the appropriate game year and look for a mod that contains this historical.mdb file
Once you have unzipped the historical.mdb file from its download it needs to be placed in its proper location. First find the folder where the game is installed. This typically would be C:/Wolverine Studios/DDSPB20xx although if you purchased the game from Steam it will have a different install path typically in C:/Program Files(x86)/Steam/steamapps/common/DDSPB20xx. Once that path is located find the setup folder and place the historical.mdb file there.
Then open the game and create a new league. When prompted during league creation select the historical option and the year you wish to start in. The game will automatically determine the proper number of teams. Finish setting up the league as you wish and when finished the game will be ready to play from that year forward. The historical replay option starts you at the proper point in time but everything from that point will be rewritten based on what happens in the game. Players will come into the league based on the data in that historical.mdb file but will be drafted by whatever team selects them and players will change teams via trade and free agency.
If you prefer a visual look at how to create a historical league we have provided a video on doing so as well.